Improvements were made in techniques developed during the last reporting period for isolating neurons from slices of the guinea pig dorsal lateral geniculate nucleus (LGNd). These improvements included the use of a new enzyme mixture (collagenase in addition to trypsin) and a better method for mechanical dissociation of the slices. The new protocol produced cells that were healthier and permitted longer whole cell recordings. Current-clamp recordings from these cells demonstrated Na + -dependent action potentials as well as low-threshold (LTS) and high-threshold (HTS) Ca2 + - dependent spikes. LTSs, which were elicited when the cell's holding potential was below - 70 mV, caused the cells to fire in a "phasic" (burst) mode. Whole cell voltage-clamp recordings revealed the existence of a fast Na + current that accounts for the upstroke of the Na + -dependent action potentials as well as three voltage-dependent K + currents. These K + currents included (i) a slowly activating, minimally inactivating, and rapidly deactivating current (IK1), and (ii) a rapidly activating, minimally inactivating and slowly deactivating current (IK2). IK1 was sensitive to tetraethylammonium (TEA), whereas IK2 was not blocked by conventional K + channel blockers. In addition, LGNd neurons expressed a rapidly activating and inactivating (transient) K + current (IA). Voltage-dependent Ca2 + currents were studied in cells internally perfused with N-methylglucamine to block outward currents. Two Ca2 + currents were characterized in these cell: (i) a high voltage activated Ca2+ current that inactivates slowly, referred to as the L-type current, and (ii) a low voltage activated, rapidly inactivating current, referred to as the T-type current. The L-type Ca2 + current was selectively inhibited by the dihydropyridine Ca2 + channel blocker nimodipine, and the T-type current was selectively reduced by low concentration of Ni2+. In current clamp recordings, we observed that both the LTS and HTS were resistant to tetrodotoxin. On the other hand, nimodipine only reduced the HTS, and low concentration of Ni2+ only reduced the LTS. We therefore conclude that the LTS is mediated by the T-type Ca2 T current, whereas the HTS is due to the L-type Ca2 + current. Ca2 + channels similar to the T-type channel in LGNd neurons are probably present in other brain regions, such as neocortex, where they may participate in epileptic phenomena. Drugs which specifically block these channels could be useful as anticonvulsants.